The work is concerned with components of the prothrombin complex. Purified bovine Factor IX is to be digested with purified thrombin to produce Factor IXa and/or an inhibitor of blood coagulation. Information is to be obtained about the properties of the inhibitor and other Factor IX digestion products produced by thrombin. In like manner, thrombin digestion of purified Protein C produces an inhibitor and peptide(s). The main product called autoprothrombin II-A (Factor XIVa) is to be studied in connection with platelet aggregation. Dog platelets and human platelets are to be used for that purpose. Bovine thrombin is to be prepared for the purpose of growing crystals to be used in molecular structure studies. Antibodies to purified bovine prothrombin, autoprothrombin III (Factor X), Factor IX are to be raised in rabbits. These antibodies are to be used for the specific removal of prothrombin, autoprothrombin III, and Factor IX from bovine plasma. It is expected that the depleted plasma can be used for the quantitative measurement of the component. The effects of prothrombin fragment 2 are to be studied with respect to inhibition of thrombin activity and combining capacity with Ac-globulin. Properties of purified Ac-globulin are to be studied. BIBLIOGRAPHIC REFERENCES: Improved Procedures for the Purification of Selected Vitamin K-dependent Proteins. Eduardo Novoa, Walter H. Seegers and Houria I. Hassouna. Prep. Biochem. 6:307-338 (1976). Primary Structure of the Amino-Terminal (Vitamin K-dependent) Region of Human Prothrombin. Daniel A. Walz, David Hewett-Emmett and Walter H. Seegers. Life Sci. 20: 79-84 (1977).